The present invention is directed towards the preparation of wood pulp for use in paper production, and more particularly, towards the brownstock washing of pulp to be used in paper production.
The production of paper begins with the processing of wood. Wood is chiefly composed of two major substances; both are organic, that is, their molecules are built around chains and rings of carbon atoms. Cellulose, which occurs in the walls of the plant cells, is the fibrous material that is used to make paper. Lignin is a large, complex molecule; it acts as a kind of glue that holds the cellulose fibers together and stiffens the cell walls, giving wood its mechanical strength. In order to convert wood into pulp suitable for making paper, the cellulose fibers must be freed from the lignin. In mechanical pulping this is done by tearing the wood fibers apart physically to create groundwood pulp, leaving most of the lignin intact in the pulp. The high lignin content of groundwood pulp leaves the paper products weak and prone to degradation (e.g. yellowing) over time. Mechanical pulp is used principally to manufacture newsprint and some magazines.
In most pulp production lignin is separated from the fibers chemically. For example, in the kraft process, wood chips are heated (xe2x80x9ccookedxe2x80x9d) in a solution of sodium hydroxide and sodium sulfide. The lignin is broken down into smaller segments and dissolves into the solution. In the next step, xe2x80x9cbrownstock washing,xe2x80x9d the breakdown products and chemicals are washed out of the pulp and sent to the recovery boiler. Kraft unbleached pulp has a distinctive dark brown color, due to darkened residual lignin, but is nevertheless exceptionally strong and suitable for packaging, tissue and toweling.
For brighter and more durable products the pulp must be bleached. In the bleaching process, the color in the residual lignin is either neutralized (by destroying the chromophoric groups) or removed with the lignin. This process traditionally has been accomplished for kraft pulp by chlorine bleaching, usually followed by washing and extraction of the chemicals and breakdown products. This process is not much different than washing clothes, the stains imbedded in cloth fibers are either neutralized by bleach, or broken down and washed out.
In current pulp production processes, the lignin solution typically undergoes two or more separate washing operations. For example, the groundwood or wood chips are first processed with chemicals under pressure and temperature, usually by either the kraft process or by the sulfite acid process. In either process, digestion dissolves the lignins thereby freeing the fibers and placing the lignin components into solution. In both processes the resulting liquid is dark in color, and the residual liquid which does not drain from the pulp and the remaining contaminants must be washed from the pulp. Further, it is desirable to recover spent liquid at as high a concentration as practical to minimize the cost of the subsequent recovery of chemicals.
Brown pulp which has been so washed retains a definite brown color and the pulp which remains is usually too highly colored for making white paper. Also, if any lignin is present, paper made from such pulp may not have a high degree of permanence and will yellow in time. Therefore, it is common and conventional to apply a bleaching process to the pulp, not only to improve whiteness, but to improve permanence of the whiteness.
The bleaching commonly is performed in a chlorination stage by applying a water in which chlorine gas has been dissolved. Other bleaching processes may be used, such as a sodium hydrosulphite process, as is well known in the art. Three chemicals that are commonly used in current bleaching operations are sodium hydroxide (NaOH), chlorine dioxide (ClO2) and hydrogen peroxide (H2O2). Bleaching may not be accomplished in a single stage and may be performed in two or more stages, each followed by washing. After bleach treatments, the pulp is subjected to a washing action to remove the water which contains the spent bleaching agents and dissolved lignin.
U.S. Pat. No. 5,275,024 shows an example of a current belt-type pulp washing machine that includes a dewatering stage (or xe2x80x9cformation zonexe2x80x9d) and multiple of counter-current washing stages (or collectively xe2x80x9cdisplacement zonexe2x80x9d). The machine employs an endless moving foraminous belt which extends about a breast roll defining an on-running end and a couch roll defining an off-running end, with a generally horizontal upper run of the belt extending between the rolls. A series of suction boxes located underneath the belt provide for initial dewatering of the pulp in the formation zone, and combine with a series of showers to provide washing and dewatering in the displacement zone.
The machine downstream from the headbox and the forming zone is divided into a series of washing zones or stages to which a washing liquid is applied from above for drainage through the mat. The freshest or cleanest washing liquid is applied to the zone nearest the off-running end of the wire and the liquid drained through the mat at that zone is collected from the suction boxes and delivered to the immediately preceding washing zone. This is repeated from zone to zone, so that the cleanest pulp is treated with the cleanest water, and the dirtiest pulp is treated with the dirtiest water.
The inventors of the present invention have recognized several deficiencies of prior tensioned belt brownstock washer belts.
In particular, the inventors have noted that current belt designs primarily include two alternative types, a high permeability, low support double layer type and a low permeability, high support single layer type. The double layer designs achieve a high drainage rate through high permeability and are appropriate for use with long wood fiber (soft wood), but exhibit sealing problems with short fiber (hard wood). The single layer designs prevent sealing with high support via low permeability, but sacrifice drainage rate.
More generally, the more open the prior art design, the less support it provides, thereby giving rise to xe2x80x9csealingxe2x80x9d problems. That is, the open prior art designs allow the fibers in the pulp mat to impinge into the belt, increasing the pulp density locally between the filaments in the belt, and thus adding resistance to flow/drainage. The machine operators can turn up the vacuums to compensate, but this increases the drag on the belt, increases abrasion and reduces service life.
In addition, the inventors of the present invention have recognized that prior tensioned belt brownstock washers such as the washer discussed in U.S. Pat. No. 5,275,024, typically employ belts that are fabricated from 100% polyamide based monofilaments (for caustic applications) or 100% polyester based monofilaments (for acidic applications); and that there are significant design deficiencies with such belts.
One of the deficiencies associated with the materials used in prior washer belts is that polyamide based fabrics tend to be dimensionally unstable in both the machine direction (MD) and the cross-machine direction (CD), which make the belts difficult to install and cause run problems as a result of growth or shrinkage outside the machine design limits. For example, MD shrinkage can result in fabrics being too short for installation while MD stretch can result in fabric lengths in excess of equipment take-up mechanisms, causing unacceptably low running tensions.
Another deficiency is that CD growth, due to water absorption, can result in fabric end to end mismatch, creating seam pinning delays, misaligned loops during pinning (weak seam), and excess width leading to excessive edge wear, abrasion, unraveling, lost production time to trim excess width, and seam rupture.
Other deficiencies include: CD shrinkage resulting in direct exposure of the pulp to the vacuum box, corrupting the basic washing process; lack of MD and CD stability, as a result of water absorption which occurs over the first few hours after installation, requiring that a break-in period be endured prior to applying stock to start washing production; the inherently hydrophilic nature of polyamide materials resulting in increased contaminant surface adhesion and a continuous decrease in drainage performance over the life of the product; and the short life span of polyester based fabrics in the presence of acidic chemical degradation giving rise to the need for adding high levels of hydrolytic chemical stabilizer to the monofilaments of the fabric.
In order to overcome the drawbacks of prior washer belts, the washer belt of the present invention is produced from a high-density multi-layer woven fabric.